In recent years, a fuel cell which is composed of a plurality of unit cells electrically connected in series has been developed. The unit cell is composed of a solid electrolyte/electrode structure sandwiched between separators, and the solid electrolyte/electrode structure is composed of a solid polymer electrolyte sandwiched between an anode and a cathode. Such a fuel cell is attracting a great deal of attention as a power source in various uses, in particular, for electric vehicles or individual power sources for domestic use because of the feature that it is a clean and highly efficient power source.
More specifically, such a fuel cell containing a solid polymer electrolyte has a proton conductive ion exchange membrane sandwiched between a pair of electrodes, an anode and a cathode, each having an electrode catalyst thereon, as a basic constitution, and a reducing agent (fuel) such as hydrogen is made to contact with the surface of the anode while an oxidizing agent (oxygen) is made to contact with the surface of the cathode to cause an electrochemical reaction making use of which electric energy is taken out from between the pair of electrodes. Ion exchange membranes of fluorine resin are widely known as the above-mentioned proton conductive ion exchange membranes superior in basic properties, while carbon papers supporting platinum thereon as an electrode catalyst are widely known as anodes and cathodes.
On the other hand, electroconductive organic polymers exemplified by, for example, polyacetylene, polypyrrole or polyaniline, containing a dopant and having an oxidation-reduction function (redox function), are watched with interest as an electrode active material for use in lithium secondary batteries (Japanese Patent No. 1,845,557), and in addition, the use of electroconductive organic polymers as electroconductive polymer capacitors having prompt discharge function is also proposed (39Th Meeting on Batteries, Preliminary Discourse, p. 173 (1998); 67Th Meeting of Electrochemical Society, Preliminary Discourse, p. 147 (2000)).
However, the electroconductive organic polymer mentioned above has low energy density when being used as an electrode active material compared with inorganic oxides such as lithium cobaltate (LiCoO2) or metals such as lithium which is presently in practical use. Thus, it is proposed that an electroconductive organic polymer is used as an electrode catalyst in order to supplement low energy density of such an electroconductive organic polymer, and that an oxidizing agent or a reducing agent is dissolved in an electrolyte in contact with the electroconductive organic polymer, thereby using a cell as if it was a fuel cell (JP-A-59-60967; JP-A-61-124070).
In such a cell, however, because both the oxidizing agent and the reducing agent are supplied as solutions, active materials diffuse into electrodes slowly so that a high output voltage is not obtained. The discharge rate is several milliamperes per square centimeter. Furthermore, the cell system is complicated and hence it is not practical.
The known fuel cell making use of a solid polymer electrolyte uses platinum as an electrode catalyst, as described above, and consequently, it is costly; leakage of acidic liquid or anode poisoning with carbon monoxide is encountered in the known fuel cell, which hinder practical use thereof. Yet a practical electrode catalyst has not been found apart from platinum.
The invention has been accomplished to solve the above-mentioned problems involved in the known fuel cells. Therefore, it is an object of the invention to provide a high output power and high voltage fuel cell which contains an electroconductive organic polymer as an electrode.